How's this for a non answer, I think you should choose whatever will best help to sell the film. I agree with Sanjit and think shooting a scene is a good option in addition to trailer and short.
I think what works the best is shooting a scene or short sequence. This was done for Sin City, and Sky Captain. It gives the investors a solid idea about what the film will look like and the quality of work you are going to deliver. I also think this is the best bang for your buck, plus it's really well done you already have one scene shot and ready to go.
The trailer, while more $$$ per shot, would also be more $$$ per second, and probably cost more over all since you will have multiple locations, and each shot needs to amazing. Trailer might be a good route if you have a mystery or complex premise that you are trying to get across. I would totally write out every detail of the trailer and make sure it leaves off a big cliffhanger.
I feel like a short film is how you get yourself noticed to make another film, especially if you're a director. If you make a short version of your feature, you run the risk of getting, "why would you need to make a feature since you can tell the story in a short".